1. Field of the Invention
The present invention relates generally to a printing apparatus performing printing of an image by ejecting a printing liquid, such as an ink or the like. More particularly, the invention relates to a suction mechanism for a printing medium at a position where the image is printed using a printing head.
2. Description of the Related Art
In general, an ink-jet printing apparatus is adapted to perform printing by ejecting an ink from a printing head toward a printing medium. The ink-jet printing apparatus is advantageous for easiness of down-sizing of the printing head, capability of printing of high definition image at high speed, low running cost, low noise for non-impact type printing system, easiness of printing of a color image using a plurality of colors of inks. Amongst, a full-line type printing apparatus which employs a line type printing head arranged with a large number of ejection openings in a width direction of a printing sheet, can further speed up printing.
However, in a full-line type color printing apparatus a plurality of line type printing heads provided for respective kinds of inks are aligned in a feeding direction of the printing sheet. In such case, a distance from the printing head located at the most upstream side and the printing head located at the most downstream side becomes significantly long. Therefore, once floating of the printing sheet is caused in the printing region, disturbance can be caused in a printed image. Also, floating of the printing sheet can be a cause of jamming or the like. Therefore, it becomes necessary to downwardly forward bias the printing sheet in order to avoid floating.
As means for biasing the printing sheet, there is a method for sucking the printing sheet utilizing electrostatic force, such as those disclosed in Japanese Patent Application Laid-open No. 133035/1995, Japanese Patent Application Laid-open No. 53081/1995 and Japanese Patent Application Laid-open No. 254460/1997. In such ink-jet printing apparatus, an electrostatic suction plate constituted of a conductive electrode is provided in a platen in the printing region to generate the electrostatic force applying a charge. The printing sheet fed from a feeding apparatus by the electrostatic force is sucked and held on an upper surface of a transporting belt and transported while printing is performed by using the printing head.
As a background art relating to the present invention, an example of the ink-jet printing apparatus will be explained with reference to the drawings. FIG. 1 is an illustration showing an overall construction of the ink-jet printing apparatus, FIG. 2 is an enlarged partial view showing the ink-jet printing apparatus shown in FIG. 1 as viewed from the above, and FIG. 3A and 3B are enlarged view of the major part in the ink-jet printing apparatus shown in FIGS. 1 and 2.
In an image printing apparatus 70 illustrated in FIG. 1, printing sheets P as a printing medium are stacked in a feeding portion 71 and are fed one-by-one from the uppermost one by a feeding roller 72. The printing sheet P thus fed is guided to a lower transporting guide 73 and is pinched between a transporting belt 74 and a pinching roller 75. The transporting belt 74 is driven by a driving roller 77 which is, in turn, driven by a not shown driving source, such as a pulse motor or the like, to transport the printing sheet P to a print start position on a platen 76.
The transporting belt 74 is stretched by the driving roller 77, a driven roller 78 and a pressure roller 79. On the other hand, in the platen, a suction force generating means 80 is fixedly mounted by adhering and is located below the transporting belt 74. It should be noted that the pressure roller 79 is rotatably mounted on one end of an arm which is pivotably mounted on the platen at the other end. The arm 83 is biased by means of a coil spring 84 for applying tension force for the transporting belt 74.
The printing head 85 is a full-line type having a plurality of printing elements arranged in alignment in a transporting direction over an entire width of the printing region of the printing sheet P. The printing heads of respective colors are arranged in sequential order 85K (black), 85C (cyan), 85M (magenta) and 85Y (yellow) from the upstream side of the transporting direction of the printing sheet P, with a given interval, and are mounted on a head holder 85a.
As shown in FIG. 2, assuming that an occupied region below the printing head 85 is S1, the suction force generating means 80 arranged below the transporting belt 74 has a size S covering the occupied region S1. As shown in FIG. 3A, the suction force generating means 80 is constituted one set of electrode plate 81 and a grounding plate 82 which are made of conductive metals. These electrode plate 81 and the grounding plate 82 are formed into comb-shape and are of the shapes mutually complement with each other, in which recessed portions of one are penetrated by projecting portions of the other. In a power supply portion 81a of the electrode plate 81, a positive or a negative voltage is applied, and a power supply portion 82a of the grounding plate 82 is connected to the ground.
As shown in FIG. 3B, in the suction force generating means 80, the electrode plate 81 and the grounding plate 82 are sandwiched by a base layer 80a and a surface layer 80b for protection. The transporting belt 72 is placed on the upper side of the surface layer 80b. The base layer 80a and the surface layer 80b are formed of synthetic resin, such as polyethylene, polycarbonate and the like.
In the construction set forth above, the printing sheet P is sucked on the upper surface of the transporting belt 74 by the suction force generating means 80 and is transported by the transporting belt 74 with printing by the printing head 85.
The printing sheet P, on which the image is printed, is sandwiched and transported by a discharge roller 86 and a wheel 87 contacted under pressure to be discharged and held on a discharged paper receptacle tray 88. The ejection roller 86 is driven by a rotational force of the driving roller 77 by not shown transmission means. On the other hand, in order to transfer a printing surface, the wheel 87 is in a shape with cone shaped tip ends so as to minimize transfer of the ink of the printed image.
On the other hand, as other construction, with similar construction as the suction force generating means provided in the platen 76, the electrode plate 81 and the grounding plate 82 are integrally provided with the transporting belt for applying a positive or negative voltage from one of side edges in the width direction of the transporting belt 74 and connecting the other side edge to the ground to form the transporting belt per se as the suction force generating means.
However, in the ink-jet printing apparatus as set forth above, the apparatus having the platen, in which the suction force generating means 80 having one set of comb-shaped electrode plate 81 and the grounding plate 82, has a region to be sucked in a size S covering the occupied region S1 of the printing head 85. Therefore, the driving motor is required a large torque in order to drive the transporting belt 74. Thus, greater motor is required. Therefore, power consumption becomes large to cause high cost in the apparatus.
On the other hand, the apparatus, in which the suction force generating means 80 is provided integrally with the transporting belt 74 per se, inherently generate a suction force over a region outside of the printing region of the transporting belt 74 immediately below the printing head 85. Therefore, the printing sheet P can subject the suction force from the transporting belt 74 even in a separating portion from the transporting belt 74 to the ejection roller 86, to make it difficult to certainly separate at the separating portion.
As set forth, since the foregoing printing apparatus generates the suction force even in the extra portion beyond that portion requiring the suction force, an unnecessarily large power can be consumed. Therefore, an improvement is desired in viewpoint of energy efficiency. Furthermore, in general, in a comb-shape electrode, when a power supply period to the electrode becomes long, the base layer 80a and the surface layer 80b of the electrode portion (particularly a corner portion 80c of the electrode) protecting the electrode may deteriorate to cause pin hole to possibly shorten a lifetime of suction force generating means. Thus, improvement of durability of the suction force generating means is desired.
On the other hand, when ink ejection is performed from the printing head using the suction force generating means, an ink droplet 148 ejected from the printing head 85 can be influenced by an electric field of the head 85 and the surface of the printing sheet P and thus charged. Especially, the ink droplet ejected from adjacent nozzles can repulse with each other to cause offset in depositing position from the predetermined depositing position to possibly cause degradation of the printing quality.